Evolutionary adaptation of the folding pathway for secretability

Secretory preproteins of the Sec pathway bear signal peptides and are targeted post-translationally to cross the plasma membrane or ER through translocases. After translocation and signal peptide cleavage, mature domains fold to native states in the bacterial periplasm or after further trafficking. During cytoplasmic transit, mature domains must remain non-folded for translocase recognition and translocation. Here, we sought the structural basis for the delayed folding mechanism of mature domains and how this is regulated by signal peptides. To address this, we compared how evolution diversified a periplasmic peptidyl-prolyl isomerase PpiA mature domain from its structural twin cytoplasmic PpiB. Using global and local hydrogen deuterium exchange mass spectrometry we showed that PpiA is a slower folder. We defined at near-residue resolution hierarchical folding initiated by similar foldons in the twins, that displayed different order and rates. Folding is delayed in PpiA by less hydrophobic/bulky native contacts, frustrated residues and a critical β -turn in the early folding region and by signal peptide-driven disorder, which disrupts foldon hierarchy. When selected PpiA residues and its signal peptide were grafted onto PpiB they converted it into a slow folder with enhanced in vivo secretion. These data reveal the structural basis of non-folding in a secretory protein, that allows its trafficking.